1. Field of the Invention
The present invention relates to an anode containing a plurality of anode active material particles and a secondary battery including the anode.
2. Description of the Related Art
In recent years, portable electronic devices such as video cameras, mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as an electric power source for the portable electronic devices, a battery, in particular a light-weight secondary batter capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a secondary battery is able to provide a higher energy density than a lead battery and a nickel cadmium battery. The lithium ion secondary battery includes a cathode, an anode, and an electrolytic solution. The anode has an anode active material layer on an anode current collector.
As an anode active material contained in the anode active material layer, a carbon material such as graphite has been widely used. However, in recent years, as the high performance and the multi functions of the portable electronic devices are developed, further improvement of the battery capacity is demanded. Thus, it has been considered to use silicon instead of the carbon material. Since the theoretical capacity of silicon (4199 mAh/g) is significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby highly improved. In this case, not only the simple substance of silicon, but also a compound, an alloy and the like of silicon are used.
However, in the case where silicon powder is used as an anode active material, a low conductive material such as an SEI (Solid Electrolyte Interface) film is deposited on the surface of silicon particles when charge and discharge are repeated. The low conductive material becomes an electric resistance and accordingly becomes a factor to lose activity of the silicon particles. Thereby, sufficient cycle characteristics are difficult to be obtained.
Accordingly, to improve the cycle characteristics even in the case of using silicon as an anode active material, various innovations have been made. Specifically, for example, a technique of forming a silicon compound on a carbon fiber disclosed in Japanese Unexamined Patent Application Publication No. 08-273660, a technique of using a metal fiber as an electrical conductor together with a silicon compound disclosed in Japanese Unexamined Patent Application Publication No. 2000-003730, a technique of covering the surface of fibrous silicon with a carbon material disclosed in Japanese Unexamined Patent Application Publication No. 2003-168426, a technique of providing an active material layer containing silicon on a fibrous conducive base material disclosed in Japanese Unexamined Patent Application Publication No. 2007-128724 and the like are known.
In addition, as a technique regarding an electrode structure, a method of manufacturing a three-dimensional network electrode using a porous resin disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 57-174484, 08-222226, and 08-225865 is also known. In this technique, after a metal film or the like is formed on the skeleton surface of the porous resin, the porous resin is removed by heating to form the three-dimensional network structure.